Marine cables are conventionally towed behind marine vessels for a variety of purposes, including sonar scanning and marine seismic surveying. It is often desirable to vary automatically the depth at which each portion of a marine cable is towed. For example, in marine seismic surveying, one or more marine cables (known as "streamers") including hydrophones are towed by a vessel. The streamers may be several kilometers or more in length. It is desirable to control independently and automatically the depth of each cable section. Often, it is desired to maintain the entire streamer in a horizontal position, so that each hydrophone is towed at the same depth. Alternatively, it is sometimes desired to maintain some sections of the streamer at shallower depths than other sections, such as to pass the streamer over a reef.
Various systems have been devised for maintaining a marine cable at a desired depth. For example, "point control" systems have been developed in which points along the cable may be raised or lowered. Examples include the systems described in U.S. Pat. No. 3,673,556, issued June 27, 1972 to Biggs and U.S. Pat. No. 3,412,704, issued Nov. 26, 1968 to Buller, et al, both of which disclose a winged device that may be attached at a selected point along the cable's longitudinal axis. Point control systems have the disadvantage that the individual depth control elements (sometimes referred to as "paravanes" or "birds") tend to cause humps or droops in the cable profile as the birds oppose the natural tendencies of a cable whose natural ballast (i.e., the cable density absent the birds) differs significantly from the water density at the cable depth. This phenomenon often causes unacceptable background noise levels even with moderately well ballasted cables.
Systems have been developed that avoid the disadvantage of point control systems, such as by adding or removing buoyant fluid (usually oil) to or from elongated sections of a marine cable (or from the entire length of the cable) to modify the cable's effective density. Conventional systems of this type, however, possess numerous limitations including the following: cable depth control procedures that are time consuming and cumbersome or likely to produce an unstable condition; the need to pre-select a cable depth at the time of cable deployment; presence of bulky pressurized fluid lines throughout the length of the cable; or the requirement that buoyant fluid be discharged into the water to decrease cable buoyancy (with attendant risk of environmental damage); or various combinations of these limitations.
For example, U.S. Pat. No. 3,909,774, issued Sept. 30, 1975 to Pavey, Jr. discloses a marine cable buoyancy control system including a buoyant fluid supply line extending the length of the cable, valve means for diverting buoyant fluid from the supply line into sections of the cable, and valve means for discharging buoyant fluid from the cable sections into the surrounding environment. A disadvantage of the Pavey system is that buoyant fluid is discharged into the surrounding environment. Another disadvantage of the Pavey system is that Pavey employs a high pressure, high flow volume, buoyant fluid supply line extending the length of the cable. Such a line would have larger diameter (possibly requiring a larger cable outside diameter), and would require higher capacity pressure maintenance equipment, than the present invention. Furthermore, should the long fluid supply line of the Pavey system develop a leak, a large volume of buoyant fluid would be discharged to the surrounding environment.
U.S. Pat. No. 3,794,965, issued Feb. 26, 1974 to Charske discloses a marine cable buoyancy control system including a pressure chamber for gas, valve means for supplying gas from the chamber to a bladder, so that the expanding bladder will expel water in the cable into the surrounding environment, and a pump for pumping water from the surrounding environment into the cable under sufficient pressure to compress the gas in the bladder, and reduce the volume of the gas therein so as to decrease the buoyancy of the cable. Alternatively, the gas may be maintained at pressure lower than hydrostatic and the pump operated to expel water from the cable interior in order to increase cable buoyancy. One disadvantage of the Charske system is that intake and discharge of water (which may be contaminated) between the cable interior and the surrounding medium tends to cause the associated valves and flow lines to clog.
Another cable buoyancy control system is descried in U.S. Pat. No. 3,375,324, issued Mar. 26, 1968 to Miller. The Miller system employs a pair of buoyant fluid supply lines extending the length of a marine cable. One line is maintained at high pressure; the other at lower pressure. Valves divert fluid from the high pressure line into an annular space surrounded by a resilient outer jacket, causing the jacket to expand radially outward, thus increasing the cable buoyancy. Other valves divert fluid from the annular space into the lower pressure line to cause the jacket to contract radially, thus decreasing the cable buoyancy. The Miller system has the disadvantages that the two fluid supply lines are bulky (requiring a larger cable outside diameter) and require high capacity pressure maintenance equipment for maintaining one line at high pressure and the other at a lower pressure. Further, the mentioned valves in the Miller system are not remotely controllable. Rather, each valve opens or closes automatically, when the internal cable pressure reaches a preset limit. This type of independent, local control system has the tendency to yield an unstable condition.
The present invention represents an improvement over conventional cable systems because it permits global, automatic buoyancy control of individual cable sections by a central control unit, without the need for any high pressure buoyant fluid flow line extending the length of the cable, without transferring buoyant fluid between the cable interior and the surrounding medium, and without the disadvantage of humps and droops inherent in point control systems.